01.11.06
The Future of Radio Communications – Today
A single system that provides CCTV from your train to a control centre, real time train status information, communication between control and drivers, a conductor with PDA relaying ticket data back to control servers and broadband internet access on-board earning revenue from passengers all at up to 160km/h – impossible you say!
Not so, the technology is available and proven.
The technology is known as MESH and has been developed, as most advanced technological solutions are, by the military for battlefield communications. Simply put, MESH provides you with mobile, ultra-reliable broadband communications whilst you are on the move.
This editorial is intended to help understand how MESH is implemented and how it works using the Motorola MEA solution.
Mesh Technology
Mesh Enabled Architecture (MEA) provides wireless broadband connectivity to fixed, portable and most notably mobile devices (more of this later). That is, each node can connect directly, or indirectly (by hopping through other nodes), with any other node in the network. The peer-to-peer nature of the mesh architecture, combined with data rate control in each subscriber and infrastructure node in the network, ensures reliable delivery while providing increased network capacity through geographic reuse of the frequency spectrum.
MEA is capable of supporting high data rate, mobile communication at vehicular speeds. The MEA system is designed to allow standard client hosts (PDA, laptop, tablet computers) with a PCMCIA interface or vehicle based systems with Ethernet connectivity to have a mobile broadband wireless connection. The MEA system works transparently to the client host in a manner similar to an Ethernet connection. MEA provides Internet Protocol (IP) based data services, as well as geo-location services and is designed to provide as much as 6 Megabits per second burst data rate in a 20 Megahertz channel to a user travelling up to 160km/h. Its geo-location for positioning Mesh subscribers in the infrastructure provides accuracy at better than ±10 meters with updates as frequent as every one second.
An MEA system can be less expensive to operate than traditional multi-platform systems and expand because it does not require a constant and significant network re-engineering effort in response to the addition and removal of buildings, growth of residential areas, and so forth. Its design enables carriers to simply add or subtract elements of the infrastructure to respond to population shifts, without the necessity of frequent planning updates and redesigning efforts.
Comparison to 802.11 WiFi
So what could you compare MEA against? The closest point of reference would be WiFi but it is important to understand that while there are some similarities between traditional 802.11 WLAN solutions and an MEA solution, there are some key differentiators between capabilities, functionality and performance that distinguish MEA deployments in key environments.
In terms of direct bandwidth comparison of a single Access Point, MEA is similar to 802.11b. The “b” standard utilises Direct Spread Spectrum Sequencing (DSSS) modulation and operates in the unlicensed 2.4Ghz spectrum. There are approximately 14 channels defined for 802.11b (and 802.11g) but only three of them are non-overlapping, meaning that channels one, six and 11 are the most commonly deployed.
Raw data rate of 802.11b is 11Mbps but the best data rate a user will typically see is around 5Mbps under ideal conditions. Sustained data rate is typically between 500kbps and 2Mbps depending on range, interference conditions and number of users on the system. Standard 802.11 Access Points are typically configured to one of the three channels mentioned previously and all communication, including control data, happens over this frequency. If your train is travelling in high density population areas, the chances are you will suffer from interference, which is not acceptable for a rail application. The key point here is that if a configured Access Point experiences interference on a particular channel, it is usually a manual exercise to reconfigure the device to use an alternative frequency.
MEA uses the Quadrature Division Multiple Access (QDMA) radio platform and operates in the 2.4Ghz spectrum and provides broadband data rates. However, unlike 802.11a/b/g solutions which are designed primarily for relatively stationary indoor deployments, QDMA was designed specifically for outdoor, high-interference and highly mobile environments. The demands for wireless communication in such harsh environments led to some of the key features that differentiate Motorola’s MEA solution in the market today. Requirements included end-to-end IP support of vehicular-speed user mobility in multi-hop networks, with or without the benefit of fixed infrastructure, as well as the ability to support broadband data rates and media-rich applications under hostile physical or RF conditions.
To this end QDMA has been optimised to support sustained data rates of 500kbps at speeds in excess of 300km/h and proven throughput of 1.5~2Mbps at speeds of 100km/h. In addition to true vehicular mobility support, QDMA leverages a dynamic multi-channel radio which enables all elements of the network, infrastructure through to subscriber, to transmit and receive on four separate, non-interfering 20Mhz wide channels. One channel carries control information such as network signalling and user coordination, network management and position-location and the other three are dedicated to carrying user data. By dynamically selecting non-overlapping channels to communicate on, MEA radios select the best channel to use on a per-packet basis allowing nearby wireless devices to communicate without interference.
In addition, an MEA solution has been design specifically for highly dynamic networking environments where assets are in constant motion, and may move in and out of network coverage or between coverage cells. For this reason network formation and recovery has been automated to ensure a high degree of tolerance to changes in the infrastructure. In an MEA network the time taken for a Subscriber to associate with a new Infrastructure device is less then 1 millisecond.
Self-forming/Self-healing
At the heart of the MEA networking technology is the Mesh Scalable Routing Protocol. MSR is designed to work efficiently without centralised wireless infrastructure equipment. This technology enables dynamic, self-forming, self-healing multi-hop routing between all MEA devices in the network. MSR utilises a proactive and reactive routing technique to maintain an efficient balance between intelligent routing while minimising resource utilisation. Further, because MSR operates at layer two it is able to communicate directly with the radio, ensuring extremely quick response to topology changes (e.g. <5ms). The ability to dynamically form network associations on a per-packet basis ensures that an MEA architecture can quickly reform or recover the network topology in response to devices joining or leaving the network.
Unlike some other wireless routing or meshing solutions, MSR takes into account not only traditional performance criteria such as bandwidth availability, but also an understanding of RF conditions.
In a MEA-based infrastructure the network dynamically adjusts itself to take advantage of available resources. Thus, if two nodes are placed close together, subscribers will dynamically associate with the node that offers the best network connectivity at a given point, providing a much more efficient use of the available resources, without the need to manually configure subscribers.
Mobility
MEA architecture is capable of fast-routing topology changes and is able to maintain connectivity even as subscribers move between base stations.
In the diagram above, the MEA-enabled tram travelling at high speed will gain a route to WR2 as it moves toward it. Eventually, the link quality to WR2 will improve over that to WR1. When this occurs the routing algorithm will determine that WR2 is a “better” path for traffic through the network. This handover is transparent to the user provided there is adequate overlap between the WR1 and WR2 coverage cells.
In intelligent transportation systems this means that data sessions between client devices onboard the train to the control centre are not interrupted as the train traverses between coverage cells. The fact that the train has learned and stored the route to WR2 before it needs it is typically referred to as a make-before-break routing algorithm and is essential in maintaining constant connectivity.
Typically, a Mesh subscriber will take in the order of 10ms to “discover” a new route to an end device and 1ms to switch over to a known route. Because MEA devices are proactive with regard to first hop neighbours, the route for the WR2 is added to its topology before it is actually required as a valid route. Thus when the subscriber makes the transition, it has already made the next route before the need to break the old one and transition time is <1ms.
Geo-Location Services
The Mesh Positioning System (MPS) offers inherent location and tracking capabilities without the need for GPS satellites. MPS takes advantage of built-in location determination methods to determine a subscriber’s position. This information is accurate to within +/- 10m and is generated in less than one second at mobility speeds of up to 300kmph. Since MPS doesn’t rely on satellites it works in both outdoor and indoor locations where GPS will not, which is useful at underground stops and for support personnel working in equipment rooms for example. Essentially a subscriber will determine its position based on time-of-flight triangulation of at least three stable reference points and relay this information to a central mapping server, as shown below.
In Summary
MESH and Motorola’s MEA™ provides:
• Seamless broadband at up to 160km/h
• Greater resilience to interference and security
• A single communication solution consolidating voice, data and telematics
• Optimised load tracking and train scheduling
• Passenger revenue generating services
• Geo-positioning with no GPS
• Automatic forming and self healing
WiFi is found in Starbucks – MESH is found on the battlefield
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